1. Field of the Invention
The present invention relates to the field of liquid crystal display, and in particular to a vertical alignment liquid crystal panel.
2. The Related Arts
A liquid crystal display is a displaying device, which comprises a layer of liquid crystal interposed between two opposite substrates. The displaying device effects light switching by applying electrical excitation and optical anisotropy of the liquid crystal. With the anisotropy of refractivity of liquid crystal molecule, the liquid crystal display applies electrical voltage to the liquid crystal molecule to reorienting the axis of anisotropy of refractivity to control the luminance of light transmitting through the liquid crystal molecule.
Whether a liquid crystal display is good or poor is primarily determined by the panel used, because the quality of the panel directly affects the impression of viewing the screen. In addition, a liquid crystal television panel takes more than one half of the cost of the whole device and is the primary factor that affects the cost of manufacturing a liquid crystal television. Thus, a good liquid crystal television is primarily dependent on the panel thereof.
The liquid crystal layer is the main body of a liquid crystal display and the liquid crystal layers used in various devices are generally formed by mixing several or tens of monomer liquid crystal layers. A liquid crystal layer generally comprises liquid crystal molecules and photo- or heat-polymerizable monomers.
In a MVA (Multidomain Vertical Alignment) liquid crystal display, without application of electrical voltage, the liquid crystal molecules are almost in the vertical orientation. However, applying an electrical voltage will cause the liquid crystal molecules to tilt in a predetermined direction set by multiple bumps and slits formed a vertical alignment film. In a liquid crystal display that incorporates a polarizer, the tilt direction of the liquid crystal molecules must also be controlled to be at an angle of 45° with respect to an absorption axis of the polarizer. However, the liquid crystal molecules, as a continuous medium, may tilt in an intermediate direction thereof and it is inevitable that liquid crystal zones that tilt at an angle different from the preset tilt angle exist. The existence of such zones causes inconsistency of luminance around those zones. To cope with such a situation of inconsistent alignment of liquid crystal, a photo- or heat-polymerizable component is added in the liquid crystal layer, whereby electrical voltage or light irradiation may be applied to cause polymerization of the polymerizable component in order to constrain the tilt direction of the liquid crystal molecule under the application of electrical voltage.
However, adopting such a technique that uses a material having a heat- or photo-polymerizable component may lead to image sticking phenomenon if the polymerization is incomplete. This is because of the polymer so polymerized having insufficient hardness and undergoing deformation caused by re-orientation of liquid crystal molecules when electrical voltage is applied.
On the other hand, if the polymerizable monomers for constraining alignment do not form sufficient cross-linked structure after polymerization, then the polymer so formed gets soft with reduced restoration power. Under this condition, when liquid crystal molecules are caused to tilt with the application of electrical voltage and sustain such a condition, even thought the applied electrical voltage is gone, the tilt angle of the liquid crystal does not return to the original condition. Such a defect also causes image sticking.
Further, when a liquid crystal display displays the same image for a predetermined period of time, image sticking also occurs in the portion that displays the image. This is deemed caused by weakened restoration power of liquid crystal molecules due to incomplete polymerization and deformation of polymer. To have the monomers sufficiently polymerized, light irradiation must be intensified or the heating time extended. This affects the manufacture cycle of product.
Several primary solutions that are proposed to cope with those image sticking phenomena occurring in liquid crystal display that contains photo- or heat-polymerizable component of monomer are discussed as follows and they also show shortcomings:
(1) For image sticking caused by insufficient cross-linked structure formed after the polymerization of the photo- or heat-polymerizable material, the solution is that a plurality of steps are carried out to irradiate the liquid crystal layer with light that has different light intensity or different ultraviolet intensity or both different light intensity and different ultraviolet intensity before or after or both before and after application of rotating voltage or under a condition of applying no rotating voltage, wherein the liquid crystal layer is a liquid crystal layer containing a photo- or heat-polymerizable material.
The shortcoming of this solution is that the number of steps that make the photo- or heat-polymerizable material contained in the liquid crystal layer polymerized to form strong cross-linked structure is large and the component and intensity of irradiation light used in each step are different. Further, a sharp border is present between the steps and parameters are set differently between these steps, such as timing for applying electrical voltage in each step, variation of light intensity between steps, or variation of ultraviolet intensity between steps.
(2) For image sticking caused by weakening of restoration power of liquid crystal molecules caused by insufficient polymerization of the photo- or heat-polymerizable material, the solution is that after the execution of the polymerization step for alignment control, additional irradiation of ultraviolet light is applied to the liquid crystal layer containing the photo- or heat-polymerizable material. The additional ultraviolet light has a wavelength that is different from that of the ultraviolet light that was previously used, before the application of the additional ultraviolet light, to polymerize the polymerizable material contained in the liquid crystal. During the irradiation of the additional ultraviolet light, no driving voltage is applied. Also, the irradiation time is 10 minutes or longer for the purposes of making the residue polymerizable material that is not completely polymerized more sufficiently polymerized.
The shortcoming of this solution is that after the step of executing light irradiation for polymerizing the photo- or heat-polymerizable material contained in the mixed liquid crystal layer, it is still necessary to apply the additional irradiation of ultraviolet light to have the polymerizable material completely polymerized and the time of the additional irradiation of ultraviolet light is extended, thereby increasing the manufacture cycle of product.